The present invention pertains to the magnetic resonance arts. It finds particular application in conjunction with compensating for eddy currents in magnetic resonance imagers and will be described with particular reference thereto. It is to be appreciated, however, that the invention may be utilized to compensate for eddy current induced fields in other applications such as diffusion or flow studies or magnetic resonance spectroscopy.
In magnetic resonance imaging, a strong substantially uniform magnetic field is generated longitudinally through an examination region. The magnetization vector of dipoles of a subject disposed in the examination region preferentially aligns with the uniform field. Radio frequency excitation pulses are supplied to cause the magnetization vectors to precess about the uniform field. Additional radio frequency pulses and magnetic field gradient pulses are applied to manipulate the precessing magnetization vector to create magnetic resonance signals, such as echo signals.
Electrical current pulses are applied to the windings of gradient field magnet coils adjacent the examination region to create the gradient field pulses. A profile, or particular temporal dependence, is selected for the current pulse in accordance with the profile of the gradient magnetic field to be applied. Commonly, the current pulses strive to approximate a square wave, trapezoid, or other ideal gradient pulse profile.
Inherently, the profile of the gradient magnetic field pulse does not match the profile of the electrical current pulse. A changing magnetic field induces eddy currents in adjacent conductive structures. The rising field at the leading edge of each gradient field pulse induces eddy currents that superimpose eddy current magnetic field components on the gradient pulse. The falling field at the trailing end of each pulse induces like, opposite polarity eddy currents that cause analogous eddy magnetic field components after the pulse. Thus, the eddy currents add unwanted eddy components to the gradient magnetic field pulse. The effect of the eddy current varies with the amount and conductivity of the material in which the eddy current is induced and the proximity of the material to the gradient coil.
In order to improve the image quality, the shape of the electrical current pulse is commonly altered such that the magnetic field produced by the sum of the current pulse and the eddy currents approximates the desired gradient magnetic field pulse profile. Commonly, a current pulse correction or pre-emphasis circuit includes a plurality of filters whose characteristic frequencies are adjustable and an amplifier with an adjustable gain associated with each filter. The filter frequency and amplification factors are adjusted to add current components whose frequencies and amplitudes effectively cancel the induced eddy current fields.
Asymmetric eddy currents are commonly dealt with by physical adjustments, such as gradient tube centering. Such physical adjustments compromise the pre-emphasis correction.
The accuracy of the eddy current compensation is affected by the accuracy with which the eddy currents or eddy fields can be measured. Various measurement techniques have been developed.
One eddy current measurement and compensation technique which is described in U.S. Pat. application Ser. No. 118,865, utilizes a search coil and an integrator. Any change in the magnetic field strength perpendicular to a plane of the coil induces a voltage in the coil which is proportional to the change in magnetic field per unit time. The voltage wave form is integrated to yield a gradient wave form which is digitized for analysis.
Although the search coil technique works well, it has some drawbacks. First, the search coil is sensitive to all magnetic field changes orthogonal to its plane, not just those changes along the magnet axes or z direction which are important in imaging. The coil is sensitive to motion, such as vibration within the main magnetic field. In order to reduce the vibration induced voltages in high field magnets, the main, uniform magnetic field may be ramped down prior to using the search coil for medium and long time constant analysis. Ramping down or turning off the main field has been found to alter reversibly the eddy current characteristics of a system. Second, the integrator circuitry tends to drift particularly during long time constant measurements. This drift or instability adversely affects the resultant measurement. Third, the search coil measures the average gradient over its area. With the relatively large search coils currently utilized, the average gradient field over the corresponding relatively large area is measured. Finer control over localization would be desirable, particularly for accurately plotting asymmetric gradient magnetic fields.
The resonant frequency of a given sample is directly proportional to the magnetic field strength. There are various NMR methods that in some way use the samples' resonant frequency to measure time-dependent changes in the magnetic field strength. The free induction decay signal (FID) from a well-defined cylinder can be used to measure a constant gradient. (J.S. Murday, "Measurement of Magnetic Field Gradient by its Effect on the NMR Free Induction Decay" J. Mag. Res., Vol. 10, pp. 111-120, 1973). The Murday method has been extended to measure short time constant eddy currents by observing the phase response of a free induction decay signal from a small, uniform sample (E. Yamamoto and H. Khono, "Gradient Time-Shape Measurement by NMR" J. Phys. E: Sci. Instrum., Vol. 16, pp. 108-111, 1986). However, spin-spin relaxation renders the Yamamoto and Khono method not applicable to medium and long time constant measurements. U.S. Pat. No. 4,698,591 to G.H. Glover and N.J. Pelc extend this methodology to measure medium and long time constant eddy current effects. One FID signal is acquired from each scan sequence and its phase evolution is calculated. The absolute phase behavior of an entire series of FIDs acquired at successive delays after the gradient is forced to be continuous in order to construct a gradient response curve. Like Yamamoto and Khono, the Glover and Pelc technique is also limited by spin-spin relaxation such that a continuous series of overlapping FIDs must be acquired to map medium or long term effects. A further weakness of the Glover and Pelc technique is that any absolute phase errors accumulate in each successive FID.
The present invention contemplates a new and improved technique for the accurate measurement of eddy currents which overcomes the above referenced deficiencies and others.